DE3433326A1 - MAGNETIC RECORDING MEDIUM - Google Patents

Description

1A-4737

TDK-242 (880025)

TDK CORPORATION Tokyo, Japan

Magnetic recording medium

The invention relates to a magnetic recording medium and a method for manufacturing the same. The recording medium has a back coating on a thin, film-like, magnetic recording medium with a thin, > ferromagnetic metal (or alloy) layer which is deposited on the substrate, e.g. B. a polyester substrate, has been applied, for example by electroplating, by chemical plating, by vapor deposition, by sputtering, by ion plating or the like Minimum.

Magnetic tapes are widely used in the audio field, the video field, and the computer field or the like. The amount of information that can be stored on a magnetic recording medium is increasing year by year. There is a growing demand for high density recording media.

A thin, ferromagnetic metal layer formed by electroplating or chemical plating or formed by vapor deposition or sputtering or by ion plating is 100% metal or a metal alloy. It is therefore suitable for extremely high recording densities. However, if a thin, ferromagnetic Metal layer is formed by such a method, so has the surface state of the substrate a considerable influence on the surface condition of the thin, ferromagnetic coating.

In order to minimize the spacing losses with respect to the magnetic head, the surface of the magnetic coating should be as smooth as possible. At the same time, the deposition of foreign materials should be avoided in order to reduce signal dropouts. In the case of a thin metal coating, the surface properties are significantly improved compared to a magnetic coating composed of a magnetic metal powder dispersed in a binder. However, since the surface roughness is only 0.01 _{μm (R 20} value, ie average value from 20 measurements, the break point being 0.17 mm in the valley step method to be explained in more detail below), the contact area is relatively large and thus the coefficient of friction is also relatively large. This makes it difficult for the belt to run smoothly. There is also a trend towards reducing the thickness of the substrate film, e.g. a plastic base film made of polyethylene

lenterephthalate, polyethylene naphthalate, polyimide or Polyamide. Attempts are currently being made with base films with a thickness of 11 μm or less. With the Reducing the thickness of the base film becomes the recording medium now so flexible that the friction increases and adhesion takes place when winding, in particular Adhesion to the guide rollers or to the support pins or the like occurs. Furthermore, the thin, magnetic metal layer to curl the medium.

When the surface is smooth to the extent described above there are even very small gaps caused by the deposition of very fine foreign materials can often lead to signal dropouts.

It is therefore the object of the present invention to avoid these disadvantages and to achieve magnetic recording? To create medium in which a thin, magnetic metal layer is provided and in which a rear Coating is provided, which develops a number of advantageous effects.

It has been found that the abrasion of the back coating as well as the curl or adhesion of the magnetic Recording medium can be reduced by having a back coating of a thermosetting Applies mass, which vinyl chloride-vinyl acetate-vinyl alcohol copolymer, Polyurethane and polyisocyanate or which includes the aforementioned components and additionally comprises nitrocellulose. Furthermore, one can use a radiation-sensitive resin composition, e.g. B. a radiation-sensitive resin that contains acrylic double bonds, Male double bonds or allyl type double bonds includes.

It was also found that when training the rear Coating with the help of a thermosetting or radiation-curable resin, the surface roughness of the Back coating is extremely important, namely the surface roughness of the thin, ferromagnetic Metal layer is extremely small and only about 0.01 µm, so that the surface roughness of the back coating should be in the range from 0.05 to 0.4μΐη, so that the S / N ratio and runnability can be improved. When the surface roughness of the back coating exceeds 0.4 μm, the properties of the surface of the ferromagnetic Coating cannot be used effectively, so that the S / N ratio decreases considerably. If on the other hand the surface roughness is less than 0.05 µm, so the friction of the back coating is too great and the running characteristics are impaired.

The back coating tends to become electrostatically charged from friction with the guide rollers. This leads to the adsorption of dust, which causes loss of signal, the winding state of the magnetic tape becomes irregular or fluctuations in output power occur. Such disadvantages can be minimized by incorporating conductive particles such as carbon black into the rear Coating. When using carbon black, however, it is easy to scrape off the same by contacting ones Parts such as guide posts for recording or reproduction or by shear stresses between the surface of the magnetic layer and the upper backing area when the tape is wound. This causes signal failures. This phenomenon is attributable to the fragility of the soot.

The aim of the present invention is to provide a magnetic recording medium in which the above-mentioned problems are as low as possible and in which in particular the abrasion of the magnetic coating and of the rear coating is minimal. For this purpose, carbon black with an average particle size in the range from 10 to 150 mpm (determined by electron microscopic measurement - this also applies below) and a powdery inorganic compound with a Mohs hardness of at most 6 and a particle size of 10 to 500 are incorporated into the rear coating mum, for example CaCO ,, kaolin, BaSO- or Fe ₂ O ₃ . It has been found that in this way one can solve the problem of the fragility of the carbon black. Under mild conditions, carbon black alone can be used without any particular problem. When used under conditions of high temperature and high humidity (for example at 40 ^° C. and a relative humidity of 80 ⁰ O, the soft, powdery inorganic compound has a balancing effect for the fragility of the carbon black, thereby reducing abrasion. Furthermore, such a powdery inorganic compound has a balancing effect for the poor dispersibility of the carbon black, which leads to an improvement in the surface properties and an improvement in the electromagnetic characteristics of the back coating.

According to the invention there is thus a magnetic recording medium created, which comprises a plastic base film and a thin, magnetic metal coating one side of the base film and a back coating on the other side of the base film. The invention is characterized in that the back coating consists essentially of a thermoset or radiation-hardened one Resin in which carbon black and powdery inorganic compound are uniformly dispersed. the

The surface roughness of the back coating is 0.05 to 0.4 µm. The carbon black has a particle size of 10 to 150 mpm, determined by electron microscopic measurement. The powdery inorganic compound has a Mohs hardness of at most 6 and a particle size of 10 to 500 mpm and the ratio of carbon black to powdery inorganic Connection is from 1: 9 to 9: 1.

In general, the back coatings, which are on the back side of a thin base film, z. B. a polyester base film, are divided into two classes. One guy acts it is a thermosetting resin composition and the other type is a radiation-curable resin composition (which is made by ionizing radiation is curable).

It has been found that the thermosetting resin composition which can be used for the back coating is a thermosetting binder which is vinyl chloride-vinyl acetate-vinyl alcohol copolymer, Contains polyurethane prepolymer and polyisocyanate. Also comes a thermosetting one Binder in question, which contains the aforementioned components and additionally nitrocellulose. Included excellent heat curing properties are achieved. In any case, the back coating contains carbon black and powder shaped inorganic compound. The radiation curable resin composition used for the back coating is to be, is a resin binder, which consists essentially of a radiation-sensitive resin, which contains radiation-sensitive double bonds, e.g. B. Acrylic double bonds, maleic double bonds or double bonds of the allyl type, and in addition carbon black and powdery ones inorganic compound. The carbon black to be used in the present invention has an average particle size of 10 to 150 mum. If the average particle size is less than 10 πιμίη, it comes to a local

Concentration of the soot, and the surface of the back coat becomes rougher and tends to have adverse effects on the magnetic layer with which it is in contact. Furthermore, the electrical conductivity deteriorates, which makes it necessary to use a larger amount. On the other hand, if the average particle size exceeds 150 mpm, the problem of abrasion of the surface of the back coat increases even though the electroconductivity is improved. The abraded particles are easily deposited on the surface of the magnetic layer and thus lead to signal dropouts. According to the invention, the problem of the fragility of the carbon black can be completely avoided by using an inorganic compound in powder form with a Mohs hardness of at most 6. Powders with a Mohs hardness of at most 6, such as CaCO. ,, Kaolin, BaSO, or oi-Fe ^ O ,, mentioned. The particle size of such a powder is generally from 10 to 500 µm. The soot does not cause any problems at normal temperatures. When combined with z. B. CaCO is, the problem of abrasion of the back coat even completely avoided under conditions of high temperature and high humidity (for example, 4O ⁰ C and a relative humidity of 80 ~ l). In addition, it is possible in this way to solve the problem of abrasion of the magnetic layer. The ratio of carbon black to the inorganic compound in powder form is from 1: 9 to 9: 1 (weight ratio). It has also been found that good results are obtained when the above components are used in an amount of 20 to 80 %, based on the total amount of the back coating.

The thin magnetic metal layer is compared to the magnetic layer, which is made of e ^ -Fe ^ O., Or cobalt-containing

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¹ X-Fe-O, made brittle. Accordingly, when an inorganic compound in powder form having a Mohs hardness of over 6 is used for the back coating, the surface of the magnetic layer tends to be susceptible to abrasion. However, according to the present invention, this problem can be prevented by using an inorganic compound in powder form having a Mohs hardness of 6 or less. In addition, this inorganic powder has better dispersibility than carbon black. Thus, by incorporating the powdery inorganic compound, the surface roughness can be reduced, the transfer of the configuration to the surface of the magnetic layer can be avoided, and the electromagnetic characteristics can be improved. This result is attributable to the fact that by using the material with a Mohs hardness of 6 or less, an adequate flexibility is achieved.

The proportions of the components of the binder for the rear coating can be varied within wide limits in the case of the thermosetting binder. In the case of a combination of a vinyl chloride-vinyl acetate-vinyl alcohol copolymer and a polyurethane, it is preferable to select 10 to 80 wt.% Of the former component, with the balance consisting of the latter component is. Further, polyisocyanate is added in an amount of 5 to 80 parts by weight based on 100 parts by weight of the total amount of the two resins mentioned above.

If one nitrocellulose is added to this binder composition, we selected 15 to 60 wt.% Of nitrocellulose, 15 to 60 wt. \ Vinyl chloride-vinyl acetate-vinyl alcohol copolymer and 10 to 70 wt.% Of polyurethane, so that a total of 100% wt. Gets . Then, polyisocyanate is added in an amount of 5 to 80 parts by weight based on 100 parts by weight of the total of the above three resins. The addition of nitrocellulose serves to further reduce adhesion and improve abrasion resistance.

If a radiation-curable binder is used for the back coating, the the following resins with radiation-curable double bonds, e.g. B. Acrylic double bonds, maleic double bonds or allyl type double bonds can be used.

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(I) vinyl chloride type copolymers

The following copolymers may be mentioned: vinyl chloride-vinyl acetate-vinyl alcohol copolymers, Vinyl chloride-vinyl alcohol copolymers, vinyl chloride-vinyl alcohol-vinyl propionate copolymers, Vinyl chloride-vinyl acetate-maleic acid copolymers, vinyl chloride-vinyl acetate copolymers, their alkyl group is OH-terminated side chains such as VROH, VYNC or VYEGX from Union Carbide Corporation (UCC), USA), and with maleic acid modified VERR (UCC).

These copolymers can by introducing acrylic, maleic or allyl double bonds after a further Processes to be explained below can be converted into radiation-sensitive resins.

(II) Saturated polyester resins

The following saturated polyester resins may be mentioned, which by esterification of saturated, polybasic acids such as phthalic acid, isophthalic acid, terephthalic acid, maleic acid, maleic acid derivatives, succinic acid, adipic acid or sebacic acid, with polyhydric alcohols such as ethylene glycol, diethylene glycol, glycerol, Propylene glycol, 1,3-butanediol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, pentaerythritol, sorbitol, glycerol, neopentyl glycol or 1,4-cyclohexane-dimethanol. The polyester resins obtained in this way can also be modified, for example with SO ₅ Na (for example Vylon 53S).

These resins can become radiation sensitive resins can be modified according to a method explained below.

(III) Unsaturated polyester resins

There are polyester compounds that are radiation-curable, contain unsaturated double bonds in the molecular chains, e.g. unsaturated polyester resins, Prepolymers and oligomers which contain radiation-curable, unsaturated double bonds and which contain the saturated Polyester resins correspond to those according to Section (II) based on polybasic acids and polyvalent Alcohols can be obtained by esterification. However, a part of the polybasic acid is by for example Replaced maleic acid.

Examples of the polybasic acids and the polyhydric alcohols for the saturated polyester resins include those compounds which have been mentioned in section (II) above, and examples of the radiation-curable, Unsaturated double bonds are double bonds which arise from maleic acid, fumaric acid or the like.

The radiation-curable, unsaturated polyester resin can be obtained by adding maleic acid, fumaric acid or the like to at least one polybasic acid component and at least one polyhydric alcohol component, followed by reacting the mixture with dehydration or alcohol removal in the usual way, ie at a temperature of 180 to 200 ⁰ C in a nitrogen atmosphere in the presence of a catalyst, followed by an increase in the temperature to 240 to 280 ⁰ C and a condensation reaction under reduced pressure of 0.5 to 1 mmHg to form the polyester resin. The content of maleic acid, fumaric acid or the like may be in the range of 1 to 40 mole-96, or preferably 10 to 30 mole-96 in the acid component

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on the degree of crosslinking, which is to be obtained in the preparation and in the radiation polymerization and so on.

(IV) Polyvinyl alcohol type resins

There may be mentioned polyvinyl alcohol, butyral resins, acetal resins, formal resins and copolymers of these components. The hydroxyl groups contained in these resins can be converted into radiation-sensitive groups by methods to be explained below .

(V) epoxy type and phenoxy type resins

As epoxy resins, resins made from bisphenol A and epichlorohydrin, methylepichlorohydrin or the like come into question, such as Epikote 152, 154, 828, 1001, 1004 and 1007 (Shell Chemical Company); DEN431, DER732, DER511, and DER331 (Dow Chemical Company); Epiclon-400 and Epiclon-800 (Dai-Nippon Ink K.K!); Phenoxy resins include PKHA, PKHC, and PKHH (UCC), these are highly polymerized resins. Copolymers of brominated bisphenol are also suitable A with epichlorohydrin such as Epiclon 145, 152, 153 and 1120 (Dai-Nippon Ink & Chemicals Co.) and the like.

The radiation-sensitive modification is effected using the epoxy groups contained in these resins.

(VI) cellulose derivatives

Cellulose derivatives with a wide variety of molecular weights can be used as thermoplastic components will. Particularly effective and preferred are nitrocellulose, cellulose aceto-butyrate, ethyl cellulose, butyl cellulose, Acetyl cellulose and the like.

These cellulose derivatives can be sensitive to radiation Resins are converted with activation of the hydroxyl groups according to processes to be explained below.

In addition to these resins, polyfunctional polyester resins, Polyester-ether resins, polyvinylpyrrolidone resins and derivatives thereof (e.g. PVP-olefin copolymers), Polyamide resins, polyimide resins, phenolic resins, spiroacetal resins, Acrylic resins with at least one acrylic or methacrylic acid ster unit with a hydroxyl group as Polymerization component and the like can be used.

Furthermore, you can use a thermoplastic elastomer or prepolymer with the aforementioned, radiation-sensitive, Combine modified, thermoplastic resin. In this way, a coating film is obtained with greater toughness or strength. Even if such an elastomer or prepolymer is modified to be radiation-sensitive is done, even better results are achieved, as will be explained in more detail below will.

In the following some examples of elastomers and prepolymers are given, which with the above-mentioned, Radiation-sensitive resins can be combined.

(I) Polyurethane elastomers, prepolymers and telomers

The use of polyurethane elastomers is particularly effective because of their abrasion resistance and their adhesion EPT films are satisfactory.

Examples of effective urethane compounds are polyurethane elastomers, prepolymers and telomers, which are made from poly-

condensates of various polyvalent isocyanates are obtained as isocyanate components and various polyesters. Isocyanate components can be used 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,3-xylene diisocyanate, 1,4-xylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 3,3'-dimethyl-4,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 3,3'-dimethylbiphenylene diisocyanate, 4,4'-biphenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, Desmodule L, Desmodule N, etc .; linear, saturated polyesters (e.g. those obtained by polycondensation of polyhydric alcohols such as ethylene glycol, diethylene glycol, Glycerine, trimethylolpropane, 1,4-butanediol, 1,6-hexanediol, Pentaerythritol, sorbitol, neopentyl glycol, 1,4-cyclohexanedimethanol, etc., with saturated, polybasic acids such as phthalic acid, isophthalic acid, terephthalic acid, Maleic acid, succinic acid, adipic acid, sebacic acid, etc.), linear, saturated polyethers (e.g. polyethylene glycol, Polypropylene glycol, polytetraethylene glycol etc.) as well as caprolactam, acrylic acid esters containing hydroxyl groups, Methacrylic acid esters containing hydroxyl groups and the like

These elastomers can be used as they are with various radiation-sensitive thermoplastic resins be combined. However, it is particularly effective to combine these elastomers with a monomer with an acrylic double bond, an allyl double bond or the like. Implement. The reaction takes place with the terminal isocyanate groups or hydroxyl groups in the urethane elastomer. on in this way, radiation-sensitive resins are obtained by modification.

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(II) Elastomers made from acrylonitrile-butadiene copolymers

Prepolymers of the acrylonitrile-butadiene copolymers are suitable which have terminal hydroxyl groups, such as "poly BD Liquid Resin" (Sinclair Petrochemicals Corp.) » or elastomers such as "Hycar 1432J" (Japanese Zeon K.K.), etc.; these are particularly cheap elastomer components; the double bonds in the butadiene structural units lead to radicals when irradiated, which cause crosslinking reactions and polymerization.

(III) polybutadiene elastomers

Low molecular weight prepolymers with tenninal Hydroxyl groups, such as "Poly BD Liquid Resin R-15" (Sinclair Petrochemicals Corp.) are particularly suitable in terms of their compatibility with thermoplastic resins. In the case of the R-15 prepolymer, it is due to the terminal hydroxyl groups of the molecule possible, the radiation sensitivity through addition of acrylic increase unsaturated double bonds at the terminal ends of the molecule. This gives prepolymers which can be used with great advantage as binders.

Cyclic products of polybutadiene such as "CBR-M901" (Japan Synthetic Rubber Co.) are also suitable. These show excellent performance when combined with thermoplastic resins. In particular, are cyclized Polybutadienes highly effective for crosslinking and polymerization by irradiation, since those contained in the polybutadiene Double bonds lead to radicals. Thus, their properties make them excellent as binders suitable.

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Furthermore, among the thermoplastic elastomers and prepolymers, the following are particularly suitable: styrene-butadiene rubber, chlorinated rubber, acrylic rubber, isoprene rubber and their cyclized products (e.g. CIR7O1; Japan Synthetic Rubber K.K.) as well as elastomers, such as epoxy-modified rubber or saturated, linear polyester with internal softness (e.g. Vylon No. 300; Toyo Spinning K.K.). These can easily be subjected to a modifying treatment in order to obtain them to make radiation sensitive. This is explained in more detail below.

In addition to these elastomers, epoxy resins, phenoxy resins or vinyl chloride-vinylidene chloride copolymers can also be used be added or cellulose derivatives.

In the following, the synthesis of the radiation-sensitive binders will be explained in more detail with the aid of examples will.

Production of tolylene diisocyanate adduct (a) Synthesis of an acrylic-modified product from a resin of the vinyl chloride-vinyl acetate copolymer type (radiation-sensitive, modified resin)

750 parts by weight of Vinylite VAOI, 1250 parts by weight of toluene and 500 parts by weight of cyclohexanone are poured into a four-necked flask with a capacity of 5 l and dissolved with heating. After raising the temperature to 80 ⁰ C., 61 parts by weight A 2-hydroxyethyl methacrylate adduct of tolylene-diisocyanate to. Further, it is 0.012 parts by weight of tin octylate and 0.012 part by weight of hydroquinone, and performs the reaction at a temperature of 80 ⁰ C in a stream of nitrogen through until these conversions

at NCO 9096. After completion of the implementation the reaction system is cooled and by adding Diluted 1250 parts by weight of methyl ethyl ketone.

Production of the 2-hydroxyethyl methacrylate (2HEMA) adduct of tolylene diisocyanate (TDI)

348 parts by weight of tolylene diisocyanate are heated in a four-necked flask having a capacity of 1 1 in a stream of nitrogen to a temperature of 8O ⁰ C. On closing added dropwise 260 parts by weight of 2-Hexamethylenmethacrylat, 0.07 parts by weight of tin octylate and 0.05 parts by weight of hydroquinone in the reaction vessel while cooling to maintain the temperature inside the reaction vessel in the region of 80 to 85 ⁰ C. to keep. After completion of dropwise addition, the mixture is stirred for 3 h at 8O ⁰ C to complete the reaction. After the reaction has ended, the reaction product is removed from the reaction vessel and cooled. 2-Hydroxyethyl methacrylate (2HEMA) adduct of tolylene diisocyanate (TDI) is obtained as a white paste.

(b) Synthesis of an acrylic modified product of a Butyral resin (radiation-sensitive, modified Resin)

A four-necked flask having a capacity of 5 liters is charged with 100 parts by weight of a butyral resin (BM-S; Sekisui Chemical Co.) together with 191.2 parts by weight of toluene and 71.4 parts by weight of cyclohexanone, and the whole is charged with heating dissolved. After raising the temperature to 80 ⁰ C tur are to 7.4 parts by weight of 2-hydroxyethyl methacrylate adduct of tolylene diisocyanate to the solution, followed by further addition of 0.015 parts by weight of tin octylate and 0.015 parts by weight of hydroquinone. The reaction is then carried out in a stream of nitrogen at 80 ^° C. until the conversion of NCO is at least 9096

has reached. When the reaction has ended, the reaction product is cooled and diluted with methyl ethyl ketone.

(c) Synthesis of an acrylic modified product of a saturated polyester resin (radiation-sensitive, modified resin)

100 parts by weight of Vylon RV-200 (Toyo Spinning Co.) is dissolved in 116 parts by weight of toluene and 116 parts by weight of methyl ethyl ketone with heating. After raising the temperature to 80 ⁰ C., is 3 * 55 parts by weight of 2-HEMA adduct of TDI and then 0.007 parts by weight of tin octylate and 0.007 part by weight of hydroquinone. The reaction is then ^{carried out at 80 °} C. in a stream of nitrogen until the conversion of NCO has reached at least 90%.

(d) Synthesis of an acrylic modified product of a Epoxy resin (radiation-sensitive, modified resin)

400 parts by weight of Epikote 1007 (Shell Chemical Co.) are dissolved in 50 parts by weight of toluene and 50 parts by weight of methyl ethyl ketone with heating. Are then added 0.006 parts by weight of N, N-dimethylbenzylamine, and 0.003 parts by weight of hydroquinone to the solution and raising the temperature to 80 ⁰ C. then added dropwise to 69 parts by weight of acrylic acid, and performs the reaction at 80 ⁰ C, until the acid number is 5 or less.

(e) Synthesis of an acrylic modified product of a Urethane elastomers (radiation-sensitive elastomer)

250 parts by weight of a urethane prepolymer of the diphenylmethane diisocyanate (MDI) type with terminal isocyanate groups (Nipporan 4040; Nippon Polyurethane Industry Co.), 32.5 parts by weight 2HEMA, 0.07 parts by weight hydroquinone and

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0.009 parts by weight of tin octylate are introduced into a reaction vessel and dissolved by heating at 80 ⁰ C. Then 43.5 parts by weight of TDI is added dropwise into the reaction vessel while cooling to maintain the temperature inside the reaction vessel in a range of 80 to hold up to 90 ⁰ C. After the end of the dropwise addition, the reaction is ^{carried out at 80 °} C. until the conversion of NCO has reached at least 9596.

(f) Synthesis of an acrylic modified product of a modified polyether elastomers with terminal urethane groups (radiation-sensitive elastomer)

250 parts by weight of a polyether (PTG-500; Nippon Polyurethane Industry), 32.5 parts by weight 2HEMA, 0.007 parts by weight of hydroquinone, and 0.009 part by weight of tin octylate are introduced into a reaction vessel and dissolved under heating at 8O ⁰ C. Then 43.5 parts by weight of TDI is added dropwise into the reaction vessel while cooling to maintain the temperature inside the reaction vessel in the range of 80 to hold up to 90 ⁰ C. When the dropwise addition has ended, the reaction is continued at 80 ^° C. until the NCO conversion has reached at least 95 °.

(g) Synthesis of an acrylic modified product of a Polybutadiene elastomers (radiation-sensitive Elastomer)

250 parts by weight of low molecular weight polybutadiene with terminal hydroxyl groups (Poly-BD-Liquid-Resin R-15; Sinclair Petrochemicals, Inc.), 32.5 parts by weight of 2HEMA, 0.007 parts by weight of hydroquinone, and 0.009 parts by weight of tin octylate Filled into a reaction vessel and dissolved ^{at 80 ° C. with heating.} Then added dropwise 43.5 parts by weight of TDI, while cooling to maintain the temperature inside the Reaktiorqgefäßes in the range of 80 to 90 ⁰ C.

to keep. After the end of the dropwise addition, the reaction is continued at 80 ^° C.,
at least 95%.

continued at 80 ⁰ C until the conversion of NCO is minor

Among the high polymer substances there are those which undergo decomposition reactions when exposed to radiation, and those which undergo molecular crosslinking upon irradiation. Among those that undergo crosslinking reactions when irradiated show include polyethylene, polypropylene, polystyrene, polyacrylic acid ester, polyacrylamide, Polyvinyl chloride, polyester, polyvinyl pyrrolidone rubber, polyvinyl alcohol and polyacrolein. Such networking Polymers can be used as radiation-curable coating resins can be used for the back layer. It doesn't require this one special should be subjected to modifying treatment, since they lead to a crosslinking reaction even without such treatment are capable.

Further, according to this method, a non-solution type resin can also be effective for the back coating can be used because it can be cured within a short time even without using a solvent can.

Electron beams, which are obtained from radiation accelerators, can be used as active energy radiation for the crosslinking of the rear layer. Furthermore, γ-rays from Co, ß-rays from ^{0 come into question}

and X-rays from an X-ray generator, etc.

From the point of view of the easy controllability of the absorption dose, the insertion into the production line and the shielding of ionizing radiation is it

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particularly advantageous to use the radiation from a radiation accelerator. From the point of view of For characteristics of the beams used, it is preferred to have an accelerating voltage of 100 to 750 KV and preferably to choose from 150 to 300 KV and an absorption dose in the range from 0.5 to 20 Mrad in order to ensure the desired penetration ability.

For the curing of the back coat, a low dose radiation accelerator (electron curtain system) from Energy Science Co., USA, is preferred from the viewpoint of lightness Can be integrated into a production line for coil coating and from the point of view of shielding secondary X-rays within the accelerator. It is of course also possible to have a van de Graaff accelerator, which has been widely used as a radiation accelerator.

It is also important that the crosslinking be carried out by irradiation in a stream of inert gas such as a stream of nitrogen gas, a stream of helium gas, or the like. It is extremely disadvantageous to carry out the irradiation in air, since ozone is formed. This has the effect that the radicals formed in the polymer are prevented from entering into crosslinking reactions.

It is therefore extremely important to work under an inert gas atmosphere, for example under N ₂ , He or CO ₂ , the oxygen concentration being a maximum of 1 #.

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The binder can also contain a radiation-curable monomer, such as acrylic acid, methacrylic acid, acrylamide or methacrylamide, as a further component.

In order to improve the abrasion resistance, the adhesion and the flexibility of the radiation-curable resin, can this can be combined with a polyurethane elastomer, a polyurethane prepolymer, an acrylonitrile-butadiene copolymer elastomer or a polybutadiene elastomer. You can also use an epoxy resin, a phenoxy resin, add a cellulose derivative or a vinyl chloride-vinyl acetate copolymer.

When the back coat is a radiation curable binder, the back coat is more uniform and tougher than a thermosettable binder.

The presence of the rear coating leads to an increase in the clinking. It refers to a fine phase modulation near the recording frequency. This phenomenon is related to the

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Smoothness of the tape run. This problem can be solved by incorporating a very small amount of one Fatty acid type or fatty acid ester type lubricants. Such an acid or ester preferably includes 10 to 22 carbon atoms. It can e.g. B. lauric acid, stearic acid, myristic acid or mixtures thereof Act. A sufficient effect is achieved if an amount of up to 3% by weight, based on the total weight the back coating.

The invention is explained in more detail below with the aid of examples. In the following examples the magnetic coating is carried out by. Vacuum vapor deposition is formed. In this case, a magnetic alloy coating is chosen which consists of 80 wt. ° s of cobalt and 20 wt.% Nikkei. It is applied by vapor deposition to the surface of a polyethylene terephthalate film, in a thickness of about 1500 X by oblique vapor deposition. The surface roughness is about 0.01 µm.

example 1

A mixture of the following composition is used for the back coating.

Parts by weight

Vinyl chloride-vinyl acetate-vinyl alcohol
Copolymer (VAGH; UCC) 30

Polyurethane prepolymer (Desmocol 22) 20

Carbon black (particle size 80 mpm) 25

CaCO ₃ (particle size 40 µm) 25

Methyl ethyl ketone .. 200

30 parts by weight of polyisocyanate are added to this mixture (Desmodule L), then the mixture is mixed. It is applied to the aforementioned polyester film,

which already carries a magnetic coating, namely in a thickness of 1.5. Then, the coating is dried, subjected to a calendering treatment and finally a heat setting. The resulting film is cut into video tapes.

The surface roughness of the back coating is varied by setting the calender treatment Values in the range from 0.05 to 0.4 μm varied.

Example 2

It will be a videotape the same way and under the the same conditions as in Example 1, but with the following composition for the rear coating is used.

" Parts by weight

Nitrocellulose (nitrocellulose, manufactured by Dicel LTD.) 30

Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (VAGH) 20

Carbon black (particle size 100 μm) 30

Polyurethane prepolymer (Desmocpl 22) 50 BaSO ₄ (particle size 80 πιμπι) 20

Example 3

Parts by weight of carbon black (20 μm) 20

Acrylic modified vinyl chloride-vinyl acetate-vinyl alcohol copolymer a) 30

Kaolin (particle size 500 µm) 25

Acrylic modified polyurethane elastomer e) 20

Solvent mixture (MIBK / toluene = 1: 1) 300

A mixture with this composition is dispersed in a ball mill for 5 hours and the mixture obtained

is applied to the rear surface of a polyester film, which already has a magnetic coating wearing. This gives a dry rear coating thickness of 1.5 µm. Then will the back coating is irradiated with electron beams under nitrogen using an electron beam accelerator of the curtain type and selects an accelerating voltage of 150 KeV and an electrode current of 10 mA. The absorption dose is 5 Mrad. The coated film is then calendered and placed on a Spool and cut into 1.25 cm (1/2 inch) wide video tape.

Example 4

Parts by weight

Carbon black C particle size 20 μm) 25

CaCO ₃ (particle size 100 ΐημίη) 25

Acrylic modified polyester resin c) 60 solvent mixture 300

This mixture is processed in the same way as in Example 3, a tape also being obtained.

Example 5

Parts by weight

Carbon black (particle size 60 mpm) 20

Kaolin (particle size 80 μm) 20

CaCO ₃ (particle size 100 µm). 20 Acrylic modified polyester elastomer e ^ 30

Vinyl chloride-vinyl acetate-vinyl alcohol * -
Copolymer (VAGH) 70

Solvent mixture 300

This mixture is processed in the same manner as in Example 3 to obtain a tape.

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Comparative examples T, 2 urid 3

In example 1, CaCO3 is replaced by the same amount of RuJS (comparative example 1). in the same way, in example 1, the carbon black is replaced by the same amount of CaCO- (comparative example 2). Furthermore, in example 1, the carbon black and CaCO _{3 are} replaced by the same amounts of Al ₂ O ₃ powder (600 nmi) (comparative example 3). The results obtained are shown in Table 1.

Table 1

Y-S / N (dB)

¹ reT exercise

; eff iz ie nt

1 run ITO run

Loosening

Signal failures "ILaufiOQ L

Abrasion d. magnetic. layer

Abrasion d.
backwards.
Coating ■

electr.
resistance
CO / cm *)

Surface, rough accuracy

For example Example2 Example 3 Example 4

Example 5

Comparative

Comparative example 2

/ match example 3

+1.0 +0.9 +0.5 +0.6 +0.6

+0.6 +1.0

0.30 0.31 0.29 0.28 0.29

0.30 0.31 0.31

0.33 0.34 0.32 0.31 0.32

0.34 0.35 0.55

50 60 50 55 50

50 250 255

100

100

80

80

75

95

350

1500

2 χ 10 "

3 χ ΙΟ "

4 χ 10

1 χ ΙΟ '

1 χ 10

2 'χ 10

2 χ 10

3 χ 10

0.15 0.12 0.28 0.25 0.30

0.32 0.20 0.50

Note: Y-S / N is a relative value based on the value of Comparative Example 3, which was set at 0 dB.

(Q) .good / O good '/ X not good draws

30- 2 # -

By running tests under conditions of high temperature and high humidity condition at '40 ⁰ C and a relative humidity of 801 the differences are clearly attributable to the type of the filler and which do not appear as clearly in a running test under normal conditions. The product of Comparative Example 1 undergoes when running test at 40 ⁰ C and a relative humidity of 80% abrasion of the back coat because carbon black was used and the coating is brittle. Furthermore, the electrical resistance is low and the ferromagnetic thin layer also has a low electrical resistance. As a result, the tape winding is hardly tightened. Since both layers have low electrical resistance, they behave slippery when being wound, resulting in an unstable winding state, and irregularities in winding state (loosening phenomenon) are observed.

In Comparative Examples 2 and 3, CaCO and Al ₂ O.? a high electrical resistance. Signal failures can therefore easily occur. Since the ferromagnetic thin film has a low electrical resistance, no loosening phenomenon is observed although the electrical resistance of the back coat is high. CaCO., Has a hardness of less than 6 and does not lead to adverse effects on the surface of the thin ferromagnetic layer. On the other hand, Al ₂ O, has a high hardness and affects the surface of the magnetic layer. Abrasion of the magnetic layer is observed.

The magnetic recording medium obtained in Example 1 was cut into a tape the width of a video tape. Leave this bond in one. VCR running at a speed of 3.8 m / s. The record and playback will be at a main frequency of 5MHz

"Ϊ433326

carried out. The S / N ratio is determined. If the Back coating surface roughness 0.4 µm exceeds, the output signal drops considerably and the S / N ratio drops by at least 1 dB. if on the other hand, the surface roughness is below 0.05 μm, the backcoat friction is too great and it is difficult to obtain smooth running of the belt.

The various characteristics were measured as follows.

Coefficient of friction

A magnetic tape is examined before the tape runs and after a tape run that has been repeated 100 times. The tape is going to placed a polished aluminum cylinder with a diameter of 4 mm, at an angle of 180 °, so that the back coating is on the inside. The belt is allowed to run at 2 cm / s. The tension on the side the tape guide and on the take-up side are measured. The coefficient of friction is derived from the measured values calculated.

Loosening phenomenon

A video tape recorder from the VHS system is used. The belt is allowed to run 100 times at 40 ° C. and a relative humidity of 80 % . Then the turns of the tape are visually inspected. The winding condition is good if no gaps are observed between the turns of the tape. This case is denoted by 0. The winding condition is inadequate if such gaps are observed between the B.and turns. This case is denoted by X.

Abrasion of the back coating

It again becomes a conventional video tape recorder. used by the VHS system. The tape a hundred times at 4O ⁰ C is allowed

and 80 % relative humidity, run. The stains in the cassette are then examined. If staining is observed, this is indicated with an X. If no stains are observed, this is denoted by 0.

Electromagnetic characteristics

1. The S / N ratio (relative value) is given for recording and playback at a main frequency of 5 MHz. A VTR from the VHS system is modified in such a way that that it can measure up to 5 MHz.

2. The signal failures are measured at 15 pS and recorded as Number / min specified.

Electron microscopic method

1. The average particle size is measured using a Transmission electron microscope determined by selecting a sample from each band.

2. A cross section is photographed using a scanning type electron microscope. In this case comes it is easy for the particles to coagulate. If the particle size varies significantly, the minimum particle size is used as the value for the average particle size taken.

Surface roughness

The surface roughness is given as an average of 20 points CR-Q) determined from a map made with

the Tallistep method (Taylor-Hobson Co.) is obtained. The break-off value is 0.17 mm; the needle pressure is

2 mg, and it is a needle with dimensions of 0.1 χ 2.5 pm used.

Claims (5)

PATENT CLAIMS 1. Magnetic recording medium with a plastic base film, a thin, magnetic metal coating on one side of the base foil and on the back Coating on the other side of the base film, characterized in that the rear coating consisting essentially of a thermoset or radiation-cured resin and carbon black dispersed therein and powdery inorganic compound and that the surface roughness of the back coating is 0.05 to 0.4 µm, the carbon black has an average particle size from 10 to 150 μm, measured with an electron microscope, the powdery inorganic compound has a Mohs hardness of at most 6 and a particle size of 10 to 500 μm and the ratio of Carbon black to the powdery inorganic compound of 1: 9 to 9: 1. 2. A magnetic recording medium according to claim 1, characterized in that the thermoset resin is obtained from a thermosetting binder, the vinyl chloride-vinyl acetate-vinyl alcohol copolymer, Contains polyurethane prepolymer and polyisocyanate. 3. A magnetic recording medium according to claim 2, characterized in that the thermosetting binder additionally contains nitrocellulose. 4. A magnetic recording medium according to claim 1, characterized in that the radiation-cured resin was obtained from a radiation-curable binder, that essentially consists of a radiation-sensitive resin consists. -...> -TB! ** lulu vfimwin ^ mm FOLLOWED 5. A magnetic recording medium according to claim A, characterized in that the radiation-sensitive resin has radiation-curable acrylic double bonds, maleic double bonds or double bonds of the AHyl type.